The prefix can be dropped from this property by eliminating -webkit-background-size (see how -webkit-opacity was handled). There are no syntactic incompatibilities so the old version of the property can just map to the new one and be eliminated.

Comment on attachment 35085[details]
Patch
r=me
You probably need to update test results for other platforms, but perhaps you can do it after you commit based on what the failures look like on those platforms’ bots.

Confusingly, in addition to the platform/mac results that this updated, there are also expected results alongside the LayoutTests themselves, in LayoutTests/fast/css/getComputedStyle/ . Should these be removed as "out of date" in favor of platform-specific ones, or the Mac ones be removed, or are things correct as-is? If the last option, these should be updated too.